This invention relates to a manipulator with a function of varying its structural compliance by adjusting the compliance of each joint.
In recent years, robot manipulators have achieved remarkable development. Generally, the conventional robot manipulator comprises arm links and joints with multidegrees of freedom so as to be able to take over human manual work. In order to enable the robot manipulator to reproduce faithfully the skillful motions a man can perform, there has been studied a manipulator control method in which means for controlling the position and means for controlling the force are employed.
In order to have such a manipulator perform a predetermined job, it is in general important not only to vary the force and position at the end effector of the manipulator but also to control the relation between the force and position, i.e. stiffness, according to the nature of the job. For example, as shown in FIG. 1, assuming that a hand 2 mounted on the front end of an arm 1 performs a peg-in-hole task for inserting a pin W into a hole H formed in a machine part, the stiffness of manipulator structure at the task coordinate axes defined on the hand can be modeled as schematically illustrated in FIG. 1. Let us assume that the stiffnesses of the robot manipulator in the directions of the x-, y- and z-axes on the task coordinate system in which the z-axis defined on the pin W grasped by the hand 2 is parallel with the central axis of the hole H are designated by variables k.sub.x, k.sub.y, k.sub.z, .alpha..sub.x, .alpha..sub.y and .alpha..sub.z. In such work, it is required that the stiffness of the arm be hard with regard to the motion in the direction of the axis of the hole and be soft with regard to the motion in the radius direction of the hole. That is, if the stiffness is soft in the x- and y-directions, a slight displacement or tilting of the pin is absorbed by the low stiffness portion, thereby to permit the pin to be flexibly inserted into the hole. In actual practice, therefore, by making k.sub.z large and k.sub.x, k.sub.y small, the pin W can be inserted into the hole H without exceeding the colliding force produced when the pin W collides against the inner surface of the hole H.
One example of such a conventional method for performing the above-mentioned work will be described hereinafter. Such a robot arm as described above is provided at its joints J.sub.1, J.sub.2 and J.sub.3, for example, with torque sensors. In addition, the arm is designed so that it can perform positional control. And, as shown in FIG. 2, the reaction F at the edge of the hole which will be caused by the interaction between the hole and the pin is detected through torque outputs .tau..sub.1, .tau..sub.2 . . . of the torque sensors provided on the joints. The amount and direction of the reaction force F can be determined by analyzing the structure of the manipulator with the torque outputs .tau..sub.1, .tau..sub.2 . . . detected at the joints. Based on the amount and direction of the reaction thus obtained, the positional commands .theta..sub.1, .theta..sub.2 . . . are successively sent to the joints of the manipulator so that the manipulator moves in the direction in which the reaction is reduced so as to move the hand 2 by small displacement along the axial direction of the hole H. As a result, by reducing the reaction force F and gradually moving the hand 2 in the axial direction of the hole H, the insertion of the pin into the hole can be accomplished.
In the above-mentioned control system, unless the detection of the reaction force F and delivery of the positional commands to the control system are carried out in very minute time steps, there is a possibility that a large reaction will be produced while the work is in progress, consequently breaking the object being worked on. In order to move it in minute steps, however, much working time is generally required and this is undesirable.